It has been recognized for many years that sodium ions in high pressure sodium (HPS) lamps, as well as ions of other elements in other lamp types, are lost by the migration of those ions through the walls of the arc-containing media in which the ionized gases are confined under electrically energized and operating conditions. The basic problem has been discussed in texts as well as some prior patents. Metals, such as sodium, which are placed within the lamps and are evaporated and driven into a gas discharge are essential for the creation and maintenance of an ionized plasma conductor which creates the light output produced by the lamp. Each type of lamp is produced with a fill or starting gas, with certain amounts of metals, halides and amalgam, and frequently with a mixture of elements, each to be operated at a selected partial vapor pressure magnitude, so that the light output will have the desired color spectrum and lumen output level when it is appropriately electrically energized. Clearly, when plasma materials escape from the discharge lamp as a result of the ion loss, the characteristics of the lamp deteriorate with color shifts and fall-off of lumen output level and are no longer in accordance with the design and operating characteristics desired. In addition, the useful life of the lamp is shortened considerably because of the drops in lamp performance and because the lamp operating voltage rises which results in undesired electrical operating changes.
While certain proposals have been advanced to mitigate this loss, a practical, effective and economical solution to the problem has not been found. Sodium loss is one of the major causes of high intensity discharge performance fall-off with operating time.